Micro-optimise screenpool event filter by caching xcb extension offset
Summary: BUG: 390499
Test Plan:
See callgrind in bug report
Added debug in the relevant section, unplugged a monitor. Saw my output
Reviewers: Plasma, broulik
Subscribers: jtamate, mart, broulik, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D10538